Electron column

The electroncolumn is made up of the gun assembly at the top, a column filled with a set of electromagnetic lenses, the sample port and airlock, and a set of apertures that can be moved in and out of the path of the beam. The contents of the column are under vacuum.

The apertures can be easily removed from the beam path by the user. This is important for operation of the objective and selected area diffraction apertures during imaging.

The apertures are located within apertures strips, typically consisting of a strip of molybdenum containing a sequence of different sized holes, that allow modulation of the beam to different degrees of precision.

Adjacent to the sample port is a ‘cold trap’; this consists of a liquid nitrogen dewar containing a conductive metal tassel which is joined to a rod at the top (see arrow path in image). The rod penetrates the column and sits near the sample. This cold area acts as a condensation site for material which leaves the sample. Such material can contaminate the chamber or affect the vacuum status of the machine.

At the base of the column is the sample viewing chamber. This has a screen which produces an image via fluorescence when impacted by the projected electrons. A set of binoculars is attached to the column and can be swung around to focus on a small, movable screen within the chamber. This screen is used for finely focusing the image. A camera is positioned either above (removable: Veleta brand in this image) or below the screen (accessed by raising the large round white screen via a lever).

The viewing port can be blocked with a light-retardant cover. It is often important to do as the phosphorescent screen is sensitive to fluorescent light sources, and can degrade when exposed to them.

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